This invention relates generally to the field of tool driving or impacting, and more particularly to a self-contained impact type wrench having a control system for accurately controlling the tension in a fastener assembly of a joint.
It is well known in the prior art that tightening a fastener to its yield point produces optimum joint efficiency. A fastened joint having a greater preload value up to the yield point of the material of the joint is more reliable and ensures better fastener performance. High fastener preload further increases fatigue resistance due to the fastener feeling less added stress from external joint loading, and dynamically loaded joints have less tendency to slip and loosen.
The prior art reveals various types of impact wrench control systems for controlling the amount of preload in a fastener. One commonly used type employs some form of torque control, in which the impact wrench tightenes a fastener to a maximum predetermined value of torque and thereupon shuts off. Examples of impact wrenches utilizing torque control can be found in United States Patents to Schoeps et al, U.S. Pat. No. 3,835,934; Hall U.S. Pat. No. 3,833,068; Schoeps, U.S. Pat. No. 3,703,933; Vaughn, U.S. Pat. No. 3,174,559; Elliott et al, U.S. Pat. No. 3,018,866 and Maurer, U.S. Pat. No. 2,543,979. Another means of controlling impact wrenches found in the prior art is commonly known as a "turn-of-the-nut" system, in which a fastener is tightened to some preselected initial condition, such as a predetermined torque value or spindle speed, and thereupon rotated an additional predetermined number of degrees before shutting off. Examples of various turn-of-the-nut impact wrench systems are found in United States Patents to Allen, U.S. Pat. No. 3,623,557; Hoza et al, U.S. Pat. No. 3,318,390 and Spyradakis et al, U.S. Pat. No. 3,011,479. Another type of control comprises imparting a constant angular momentum of each impulse blow, such as found in the United States Patent to Swanson, U.S. Pat. No. 3,181,672.
In all of the control systems described in the above-noted patents, prior knowledge of the fastener and joint characteristics must be known or assumed in order to determine either the exact predetermined final torque, the exact amount of additional rotation, or the amount of constant angular momentum of each impact blow. It is well known that tightening to a predetermined preload condition, such as the yield point, is a function of many variables, among them being joint stiffness, fastener stiffness, surface friction between mating threads and thread form of the mating threads. Therefore, if it is desired to tighten an assembly to the yield point with any of the systems described in the above-noted prior art patents, this point cannot always be accurately determined because the conditions of each fastener assembly and joint can vary and may not be known in advance. This consequence can lead to uneven tightening from joint to joint in the same structure, which, in turn, can result in loosening of one or more fastener assemblies, especially in the presence of vibrations, and premature fatigue failure.
In the control system described in co-pending application, Ser. No. 818,511, which is a continuation of co-pending application No. 579,110 now abandoned, the yield point of a fastener assembly is attained by a tightening apparatus which periodically applies a tightening moment to the fastener assembly, such as, for example, an impact wrench. The yield point is attained when an instantaneous peak moment signal has not increased by more than a predetermined amount over a stored previous peak moment signal. While this control system works satisfactorily in certain applications, the present invention is seen as being an improvement thereover.